6.5 Protists 2 Flashcards
Cryptosporidium spp.; General Features; host choice, size, human health
- cosmopolitan parasite that infects a wide variety of hosts, usually the young or immunosuppressed
- important in neonatal cattle
- smallest parasite commonly diagnosed
- zoonotic threat
Cryptosporidium spp.; Life Cycle
- typical coccidian (apicomplexan) life cycle
- all stages of develop in an epicellular location
- oocysts sporulate within the host in contrast to many other coccidia
- both thick-walled and thin- walled oocysts formed which gives rise to autoinfection
Cryptosporidium spp.; most important point about infectivity
autoinfection
Cryptosporidium spp.; Pathogenesis/Clinical Signs
- large numbers infect the mucosal surface of the small intestine causing villus atrophy, malabsorption and enteritis
- non-bloody, watery diarrhea most typical (a “cholera-like diarrhea”)
- diarrhea lasts 1 - 3 weeks in normal animals but prolonged in immunocompromised
- greatest threat is dehydration in neonates
Cryptosporidium spp.; Diagnosis
- look for small, refractile oocysts in feces
- Acid fast staining of smeared feces
- IFA of fecal smears
- diarrheic stool is examined using the “OVC Puddle Technique” (an on-slide flotation method) - oocysts refract a pink colour
- Fecal antigen tests for human Cryptosporidium are now available but utility in domestic animals not well documented
Cryptosporidium spp.; Treatment
- no effective drug therapies
- treat the symptoms using fluid replacement
- animals are usually immune to future infections
Heteroxenous Coccidia; General Comments; host preference/phases
- parasites in this group usually infect animals that are in a predator-prey relationship
- definitive (final) host has the intestinal phase
- intermediate host has the extra-intestinal phase(s)
- dogs and cats are usually the definitive hosts in such life cycles and therefore host the intestinal development of the parasite
Heteroxenous Coccidia; Family Sarcocystidae members and hosts
Sarcocystis spp.
* one defintive host; one intermediate host
Toxoplasma gondii
* cats are definitive hosts; many intermediate hosts
Neospora caninum
* definitive host unknown; many intermediate hosts
Cystoisospora spp. (discussed last module)
* one definitive host (carnivore or omnivore); optional paratenic host (encysted sporozoites only)
Coccidia – General Life Cycle
Oocysts containing sporozoites (Environment)
> Merogony >
Meronts (=schizont) containing merozoites
> Gametogony >
Gametes:
-Macrogametes - female
-Microgametes - male
-fuse to form a zygote
> Sporogony >
Oocysts containing sporozoites
Heteroxenous Coccidia – Life Cycle
Cycle is divided based on location:
ENVIRONMENT
Oocysts containing sporozoites
INTERMEDIATE HOST
> Merogony >
Meronts (=schizont) containing merozoites
DEFINITIVE HOST
> Gametogony >
Gametes:
-Macrogametes - female
-Microgametes - male
-fuse to form a zygote
> Sporogony >
ENVIRONMENT
Oocysts containing sporozoites…
Sarcocystis spp.; General Characteristics; hosts
- highly host specific, both final and intermediate in most cases
- many different Sarcocystis species that infect predatory reptiles, mammals and birds of prey as definitive hosts
- herbivorous or omnivorous tetrapods act as intermediate hosts
Sarcocystis spp.; General Characteristics; infection site, pathogenicity
- infect the intestinal tract of the definitive host
- infect endothelium and muscles of the intermediate host
- usually non-pathogenic in the definitive host
- may cause disease in intermediate host
Definitive host vs Intermediate host
The definitive host is the one which harbors the adult parasite and where the parasite reproduces sexually. The intermediate host is the host which harbors the larval stage or the asexual forms of the parasite.
Sarcocystis spp.; Life Cycle
- similar to Cystoisospora spp. but split into two hosts
- oocysts or sporocysts released in feces of final host
- these infect only the intermediate host
- 2 cycles of merogony occur within vascular endothelial cells before the parasites enter muscle or nerve cells where they form a cyst
- large cyst or “sarcocyst” containing bradyzoites formed - these may last for years
- when eaten by the definitive host, the bradyzoites penetrate the intestinal epithelium and develop into gametes which fuse to form a zygote
- the oocyst sporulates within the host and is shed in the feces fully sporulated
- oocyst often breaks to release sporocysts in feces
Sarcocystis spp.; Pathogenesis/Diagnosis
- non-pathogenic for definitive hosts
- vascular phase in intermediate hosts may cause significant disease
- sarcocysts are not normally pathogenic
- diagnosis is made in definitive host on finding oocysts or, more commonly, sporocysts in a normal fecal flotation
- in intermediate host, infrequently diagnosed except in case of Equine Protozoal Encephalitis
Equine Protozoal Encephalitis; Life cycle
Sarcocystis neuronia sporocysts from opossum feces
>ingested by armadillo and other natural intermediate hosts
>sarcocystis neuronia sarcocyst in muscle
>muscle tissue of EPM-infected carrion ingested by oppossum
>Opossum is definitive host
> Horse can ingest sporocysts from opossum feces
aberrant host
Equine Protozoal Encephalitis; General Features/Pathogenesis/Treatment
- horses a presumed aberrant host
- opossum is definitive host
- ingestion of sporocysts – vascular invasion and finally CNS of horses
- no conversion to sarcocysts – extended replication and associated tissue damage in brain and spinal cord
- clinical signs of progressive asymmetrical ataxia with muscular atrophy – confirm active infection with CSF sample for Western Blot or PCR
- treatment pyrimethamine/sulfas – long term therapy
Toxoplasma gondii; General Characteristics; hosts, range, severity
- similar to Sarcocystis spp. except for the range of intermediate hosts
- felids are the only definitive hosts
- widespread parasite because of its ability to both move from intermediate host to intermediate host
- serious pathogen during extra-intestinal phase
Toxoplasma gondii; Life Cycle
- same apicomplexan life cycle as Isospora spp. split into intestinal and extra-intestinal cycles
- most similar to Sarcocystis
- forms oocysts in intestinal tract of
only cats that resemble Cystoisospora - oocysts are infective to both cats and most other animals
EXOGENOUS
Excretion of unsporulated oocyst
>sporulated oocyst
INTERMEDIATE HOST (nonfeline vertebrates)
Ingestion
>excystation
>tachyzoites
-vertical transmission during pregnancy
-tissue cysts formation
-carnivorism, by another intermediate host or feline
DEFINITIVE HOST
Ingestion of tissue cysts via carnivorism, or of sporulated oocysts due to fecal contamination
»GI tract
>release of zoites
>merogony
>gametogony
>fertilization
>excretion
- Intestinal or Enteroepithelial cycle. This occurs in cats and members of the cat family and is generally similar to that described for Isospora. Merogony leads to gametogony, resulting in the production of Toxoplasma oocysts. These are unsporulated when passed in feces and undergo sporogony to give rise to sporulated oocysts that are morphologically similar to those described for Isospora (see Module 12.4). These are infective to all susceptible hosts and can
start a new life cycle. - Extra-intestinal cycle. Two additional stages occur in the extra-intestinal tissues of cats and constitute the entire life cycle in other mammalian and avian hosts. These 2 stages, the group or multiplicative stage containing organisms termed tachyzoites, and a cyst stage containing organisms termed bradyzoites, comprise the extra-intestinal cycle. In cats, both the intestinal and the extra-intestinal cycles start almost simultaneously. In the extra-intestinal cycle parasites develop within a multitude of cell types - fibroblasts, reticular cells, hepatic parenchymal cells, pneumocytes, myocardial cells and neurons. Parasites divide within these cells till 8 to 16 or more tachyzoites accumulate. These parasitized cells then disintegrate and new cells are infected by the tachyzoites. After several cycles of this stage, called the group stage, the tachyzoites enter new cells, mainly in the brain, heart and skeletal muscle. Here they multiply to give rise to the cyst stage. These are called Toxoplasma tissue cysts and are characteristic of chronic infections. Cysts contain bradyzoites and may persist for the life of the host. These cysts are infective to all susceptible animals, including cats, and can start a new life cycle in a manner similar to infective oocysts.
Toxoplasma gondii; Pathogenesis - Cats
- intestinal phase non-pathogenic - no diarrhea
- extra-intestinal phase in young kittens
- replication of group stages in lungs primarily (causes pneumonia) and elsewhere in severe infections (eg. liver, muscle, heart and pancreas)
- depression, anorexia and rapid death in severe cases
Toxoplasma gondii; Pathogenesis - Intermediate Hosts; Acquired toxoplasmosis
Acquired toxoplasmosis
Humans:
- lymphadenopathy esp. cervical nodes - encephalitis*, myocarditis, pneumonia
- can be fatal in immunocompromised
Animals:
- not usually diagnosed
- widespread based on serology
- young animals may become clinical
Toxoplasma gondii; Pathogenesis - Intermediate Hosts; Congenital Toxoplasmosis
- most serious form of toxoplasmosis
- common cause of abortion and neonatal death
- causes focal inflammation and necrosis of fetal cotyledons - whitish nodules up to 2 mm
- group stages and cysts found by histology
- fetus often demonstrates lesions and organisms in the brain and liver
Toxoplasma gondii; Pathogenesis - Intermediate Hosts; general types
Acquired, congenital
Toxoplasma gondii; Zoonotic Threat
- humans susceptible to both oocyst-derived and tissue cyst derived infections
- immunocompromised - acquired toxoplasmosis
- primary infection during early pregnancy results in severe congenital toxoplasmosis hallmarked by fetal encephalitis with stillbirths relatively common
- most congenital infections cause some permanent neurologic sequelae
Toxoplasma gondii; Diagnosis
- oocysts can be found in fecals from cats (rarely)
- usually diagnosed by serology
- elevated IgM titres in the absence of significant IgG is evidence of an active or recent infection
- low IgM titres and higher IgG titres indicate past exposure to the infection
Toxoplasma gondii; Treatment
- most animals do not need treatment and many diagnoses do not occur quickly enough to allow treatment
- congenital toxoplasmosis is not treatable
- acquired toxoplasmosis may respond to a combination of triple sulfonamides and pyrimethamine (all off-label usage)
Neospora caninum; General Comments; hosts, effects
- closely related to Toxoplasma gondii
- definitive host – dogs
- asexual development known from a variety of hosts
- neurotrophic
- most important in dogs (causes ascending paralysis) and cattle (causes abortions)
Neospora caninum; life cycle with cattle
Oocysys passed in feces, contaminate cattle ration
> cross-placental transmission infects fetus
Option 1
»Abortion
>infected carcass
>ingested by hypothetical intermediate host
>ingested by canid definitive host
>Oocysys passed in feces, contaminate cattle ration
Option 2
»Birth of infected heifer with tissue cysts in brain
>maturation and breeding of congenitally infected heifer
>repeat option 1, option 2, OR give birth to unaffected calf to break the cycle
Only merozoites (tachyzoites and bradyzoites) are found in ruminants (intermediate hosts).
Tissues cysts with clusters of merozoites are found principally in neural tissues of infected intermediate hosts. These encysted bradyzoites are believed to be infective to dogs in which
the parasite initiates oocyst production.
- vertical transmission most important in dogs and cattle
Neospora caninum; transmission
- transmitted transplacentally to more than one litter of pups or more than one calf
- In pups may have neurological signs, especially hind limb and ascending rigid paralysis
- In cows, abortion is frequent but many calves are born infected but clinically normal – unlike Toxoplasma, can cause repetitive abortions in the same cow
Neospora caninum; Diagnosis
- ascending paralysis in young dogs (esp. litters)
- organisms in CSF or biopsy
- parasite-specific ELISA to establish herd status in cattle
- herd prevalence in Canada (2005 figures): 38-90% in dairy, 36-78% in beef
Neospora caninum; Treatment/Prevention
- drugs are currently undergoing testing
- sulfonamide treatment suggested for puppies but no treatment for calves
How worried should a pregnant woman be about toxoplasmosis? should she get rid of her cat?
- should only be worried if she gets infected for the first time during pregnancy; if she has had the infection before, there is nothing to worry about
- cat is only a risk during the time it sheds the oocysts, which lasts about 2 weeks of the cats entire lifetime. Rarely, the cat will shed again if its immune system becomes compromised.
- oocyst needs about 4 days to sporulate after being shed, so proper hygiene practices/ cleaning cat’s litter regularly mitigates this risk
- indoor cat would have to be infected for the first time to start shedding; would likely have to eat an intermediate host raw, which is unlikely for an indoor cat
- woman and cat can have screening test if worried
N. caninum control measures
Now that the dog has been shown to be a definitive host for N. caninum it is prudent to prevent their access to placentas, aborted fetuses and dead calves. One should also eliminate opportunities for dogs to defecate in feed and bedding. However, since vertical transmission is considerably more common than horizontal transmission on most farms, control efforts should be focused at minimising the N. caninum herd seroprevalence. Screening of cattle sera for
antibody to N. caninum is an effective means to identify animals for culling, or for identifying negative replacement heifers. For herds with a high seroprevalence, farmers may cull selectively over multiple years, initially focusing on seropositive animals that have aborted.
